Answer:
Relative velocity simply means velocity relative to a certain observe point, in this case, the river shore.
This is a type of complex movement; a boat is moving with a certain velocity, let's say 8 m/s. However, the river this boat moves on also has its velocity, let's say 2 m/s. Since the shore isn't moving, the velocity of the boat relative to the shore will be simple resultant velocity of the boat's and the river's velocities.
Let's illustrate this with two examples:
1) if the boat is moving downstream both are moving in the same direction, so the resultant will be 8 m/s + 2 m/s = 10 m/s.
So, any spectator looking at the boat from the shore will think its velocity is 10 m/s, when in reality it's 8 m/s.
2) if the boat is moving upstream, these two velocities have opposite directions, so the resultant will be 8 - 2 = 6 m/s
So, as a short summary, relative speed is an object's speed in regard to another chosen referent point.
